Skid steer loaders were first invented about 30 years ago to fill a need for a small highly maneuverable vehicle that was capable of carrying an implement mounted on loader arms. Skid steer loaders are typically small vehicles, on the order of 10 to 14 feet long, that rest on four or more wheels, at least two of which being disposed on each side of the vehicle.
In order to turn these vehicles, the wheels on opposing sides of the skid steer loader are driven at different speeds. This causes the faster moving wheels on one side to advance that side over the ground faster than the other side on slower moving wheels. The effect is to turn the vehicle toward the wheels on the slower moving side. Since the wheels are not turnable with respect to the vehicle, the vehicle turns by skidding slightly, hence the name “skid steer loader.” In the extreme case the wheels on one side of the vehicle can turn in the opposite direction at the same speed as the wheels on the other side of the vehicle. When this mode of operation is selected, the skid steer loader will rotate in place about a vertical and generally stationary rotational axis.
This ability to change direction by rotating about an axis within the footprint or perimeter of the loader itself was the primary reason why the skid steer loader achieved its great success.
This mode of turning by skidding permits the skid steer vehicle to operate within confined spaces to provide workers within those spaces the added power that a mobile lifting arm or blade can provide.
The skid steer vehicle is used inside buildings that are under construction or are being fabricated. The skid steer vehicle can carry material and tools quite close to an inside work location right to where workers are fabricating the building, making modifications to the building or other related work. Larger vehicles that have lifting and load-carrying abilities, such as bulldozers, backhoes, front wheel loaders and the like do not have the same ability.
Backhoes with their large rear tires and wide stance cannot easily go through doorways or apertures of buildings that are under construction. Furthermore, their stance is typically too wide to pass through the doorways and their height as well is too large, typically on the order of 10–12 feet off the ground—too large to pass through building doorways or wall openings. Backhoes turn by steering their front wheels with respect to their chassis, giving them a typical turning radius of 25–45 feet, still much too large to use conveniently inside a building.
Wheel loaders have an extremely wide stance and large bucket, permitting them to carry and move large loads at relatively high speeds over broken ground. Wheel loaders are intended for such locations as road construction sites, rock quarries, steel mills and other outside locations where large capacity, relatively high speed vehicles are beneficial.
Articulated wheel loaders are also constrained by their method of steering: they include two-piece chassis that bend slightly in the middle permitting them to turn in a circle with a radius of about 30–50 feet. This would require an extremely large area in which to turn around, and they would be dangerous in crowded work areas. With a height of about 10–15 feet, they cannot pass through opening or doorways to be used inside buildings to carry tools and supplies and support inside workers.
The only truly practical work-horse for in-building work and work in close proximity to workers is the skid steer loader, and it has been used for those purposes for many years.
One big advantage to skid steer loaders is their low height, typically no more than 8 or 9 feet. This is low enough to permit the vehicle to pass through doorways. A further advantage to skid steer vehicles is their narrow width. They are typically less than six feet Wide, permitting them to pass though double door ways into commercial buildings under construction. In this manner, they can easily ferry tools and material from larger vehicles and storage areas outside the building into the building itself where they can be delivered to the workers.
Another beneficial feature of skid steer vehicles is their ability to turn in place. By turning in place, skid steer vehicles can often avoid backing up at all when inside a building permitting them to maneuver quite carefully through and around work stations, workers, and piles of materials when moving about inside.
Two significant drawbacks to skid steer vehicles are their drive systems and their rigid suspensions.
Skid steer vehicles typically use chain drives to connect hydraulic motors to wheels. These chains are placed in enclosed “chain tanks” that are filled with liquid lubricants. Hydraulic motors extending into the tanks engage chains that extend forward and aft to the forward and the rear drive wheels.
Each side of the vehicle has a chain tank with its associated motor and drive wheels, the chain tank extending fore-and-aft along each side of the vehicle.
As the vehicles are driven, the rapid back-and-forth of the drive systems tighten and slacken the chains repeatedly. Each time the hydraulic motor is reversed to change the vehicle's direction, the chain is slackened in one direction and jerked taught to begin traveling in the opposite direction.
This repeated jerking of the chains and the high loads provided by the hydraulic drive motors cause the chains to wear rapidly and require replacement frequently. The down-time to replace the chains, the cost of disassembling and reassembling the vehicles and the cost of new chains themselves, all add significantly to the total cost of ownership of a skid-steer vehicle.
Chain drive skid steer vehicles are typically unsprung. The wheels of the vehicles are most commonly supported on fixed rotating axles that extend outward through the sidewalls of the chassis. The sidewalls of the skid steer chassis itself are solid steel plates having apertures through which the axles pass. The axles, in turn, are supported on bearings that are fixed to the side walls.
The unsprung nature of the skid steer vehicles poses a problem in itself, since it prevents the skid steer from absorbing the shocks caused by travel over irregular terrain. The short wheelbase and narrow width of a skid steer compounds the problem. At speeds above about 8–10 miles per hour over rough terrain, the skid steer vehicles tend to pitch and roll excessively. For this reason, skid steer vehicles tend to be continuously operated at speeds not exceeding six miles per hour.
Given their difficult performance at speeds much in excess of six or eight miles per hour, most skid steer vehicles are speed-limited by design. They are engineered to go at a maximum speed of 10–12 miles per hour, which can be considered a general practical ground speed limit for an unsprung skid steer vehicle, even one traveling over even ground.
Six to eight miles per hour may seem a satisfactory rate of travel. For many work environments, however, it is not satisfactory. In many work environments a skid steer vehicle may have to repeatedly travel 200–400 feet, carrying buckets of soil or other materials. At 6–8 miles per hour, this distance can be quite large. In other environments, such as road construction, a work site can be several miles long. These distances pose a significant problem to the mobility and usefulness of a skid steer vehicle and sorely limit its usefulness and applicability.
Several ways of solving the dual problems of chain wear and speed have been individually proposed. In the Related Applications, the inventors have proposed using a right angle gear drive with a fore-and-aft extending driveshaft to couple the hydraulic motor to its front and rear drive wheels.
While the illustrated arrangement alleviates the wear problem by replacing the chain with a driveshaft/gearbox system, it does not provide a sprung suspension, since the forward and rear axle housings that support the front and rear axles are fixed to the chassis. The vehicle is unsprung.
In the skid steer vehicle shown in U.S. Pat. No. 6,584,710, a sprung suspension is provided, which alleviates the problems due to lack of springing, but requires additional components: two more hydraulic motors, four reduction gear drive hubs, and the hydraulic conduits required to connect these components together.
What is needed is a skid steer vehicle that provides both the smooth ride provided by a sprung suspension and the durability and low maintenance of a driveshaft and gear drive system.
What is also needed is a skid steer vehicle with a sprung suspension and a chainless drive system that does not require a separate motor for each driven wheel.
It is an object of this invention to provide such a system in one or more of the embodiments described and claimed herein.